Vehicle wheel bearings with non powered inner races or spindles have the advantage of being able to be sealed on one side by a one piece end cap, which is a non rubbing, friction free seal. The disadvantage of an end cap is that it has a relatively large surface area. The inner surface of the cap is exposed to the internal bearing pressure, and the outer surface to ambient pressure. Each pressure may fluctuate relative to the other. If the internal pressure is higher, the cap is subject to expansion, which may tend to dislodge it. If the ambient pressure is higher, the end cap is subject to contraction that may be sufficient to temporarily break the seal with the hub and pull outside contaminants in. Temperature fluctuations also occur.
Conventional end caps are typically steel stampings which are rigid enough to resist any pressure fluctuations. Steel end caps also generally match the steel hubs to which they are secured in terms of temperature caused expansion and contraction. Newer cap designs save weight and expense, as well as providing better corrosion protection, by molding the end cap from plastic materials, which are also much more flexible and elastic than steel. This flexibility and elasticity can be used to advantage to provide integral venting mechanisms that relieve pressure before the cap can be dislodged. One example, disclosed in co assigned U.S. Pat. No. 5,172,984 to Lederman, provides slots in the cap that are blocked by the edge of the hub under normal pressures. When the cap expands or contracts, the slots become unblocked to allow pressure in or out. Another example, disclosed in co assigned U.S. Pat. No., 195,897 to Lederman, incorporates a small, deliberate fracture line at the center of the cup that can break and open in response to pressure fluctuations, and re close when pressure returns to an equilibrium.